1. Technical Field
Embodiments of the present disclosure relate to improved compositions for de-icing surfaces and to anti-icing compositions intended to prevent icing of surfaces in freezing conditions. In some embodiments, the compositions of the present disclosure are further suitable for preventing the build-up of snow or other frozen or freezing precipitation on surfaces. In some embodiments the compositions of the present disclosure can provide enhanced de-icing and anti-icing performance and enhanced environmental benefits in comparison to conventional products. In some embodiments the compositions of the present disclosure can usefully be applied to surfaces traversed by vehicles and by pedestrians, for example. In some embodiments of the present disclosure the compositions of the invention can usefully be applied to ground surfaces traversed by aircraft.
2. Background of Related Art
Chemical de-icing and anti-icing treatments are routinely used in freezing weather conditions, notably in the winter and in cold climates, to prevent the formation or build up of snow and/or ice on surfaces traversed by pedestrians or vehicles, such as on paths, pavements (sidewalks), stairways, roads, airport taxiways, aprons and runways and such like. Such compositions act by melting existing snow and ice, or by preventing the formation of ice, on the surface to which the treatment is applied.
The use of de/anti-icer compositions at airports imposes particular constraints on the nature of the compositions in terms of their effectiveness, their compatibility with aircraft (for example the compositions must not cause corrosion or other damage to any part of the aircraft which might have a detrimental effect on the airworthiness of the aircraft) and, more recently, in terms of the environmental effects of the compositions.
Many prior art de/anti-icer compositions for use on travelled surfaces have contained ethylene glycol. Although ethylene glycol-containing compositions demonstrate high performance with regard to de-icing and anti-icing ability, they also suffer from several significant disadvantages. Any de/anti-icer composition applied to a travelled surface is likely to be washed off the surface by water from melting snow and ice and/or by rainwater. The de/anti-icer composition is thus brought into contact with the wider environment. Ethylene glycol is toxic to humans with numerous cases of poisoning reported in the UK and worldwide. Furthermore, ethylene glycol-based de/anti-icers have a high Chemical Oxygen Demand (COD) and thus exhibit deleterious effects when exposed to the wider environment following their application. As a result, the use of ethylene glycol has been prohibited in aircraft de-icing fluids in Europe and at some airports in North America.
Alternative de-icer compositions include those with urea as the active ingredient. However, urea-based de-icer compositions have a highly adverse environmental impact due to their very high COD and by acting as a rich source of nitrogen. Urea-based de-icer compositions are therefore highly damaging to any watercourses which receive run-off from the surface to which the de-icer is applied. In addition, the suitability of urea-based compositions as effective de-icers is further compromised by a comparatively high minimum effective temperature of −12° C. (10° F.) which is insufficiently low in colder climates where lower ground temperatures are often encountered.
In an effort to overcome the disadvantages of ethylene glycol-containing and urea-containing products, de/anti-icing compositions based on 50% w/w aqueous solution of potassium acetate are now used at airports in cold winter climates around the world. Potassium acetate solution can be combined with suitable corrosion inhibitors and meets the high standards of non-corrosiveness to aircraft materials required for airside use, and the COD and Biological Oxygen Demand (BOD) load in water run-off generated by potassium acetate based products is much lower than ethylene glycol or urea-based compositions.
Airside de/anti-icing compositions based on 50% w/w aqueous solution of potassium formate are also used and function in the same way as 50% w/w potassium acetate based fluids but have even lower COD and BOD values.
It is generally considered advantageous in formulating liquid de/anti-icer compositions to seek a composition having the lowest possible freezing point to maximise the de/anti-icing performance of the composition. Chemical de/anti-icer compositions act as freezing point depressants and function by introducing the freezing point depressant into contact with the body of frozen or liquid water to which the de/anti-icer composition is applied. The freezing point depressant lowers the freezing point of the body of water. When the freezing point in a water-based system is below the ambient environmental temperature, frozen water is melted (resulting in a liquid solution of the freezing point depressant in water) and initially liquid water forms a solution of the freezing point depressant and is prevented from freezing.
The freezing point of a de/anti-icer composition based on water and a freezing point depressant is related to the concentration of the freezing point depressant. In most cases, as the concentration of the freezing point depressant is reduced by the introduction of more water, for example, by falling precipitation and/or by melting of ice or snow, the freezing point of the water-de/anti-icer composition mixture rises. That is, the resultant mixture of water and de/anti-icer composition freezes at a higher temperature than a corresponding mixture where the de/anti icer composition concentration is higher. If by the introduction of more water the freezing point of the mixture of de/anti-icer composition and water becomes equal to or greater than the ambient environmental temperature, the de/anti-icing composition ceases to be effective: the mixture will freeze. Consequently, with a mixture in this state, treated surfaces will freeze over or no further effective clearing of frozen water will occur unless further de/anti-icing composition is applied.
In view of the above considerations, potassium acetate or potassium formate based de-icers, are formulated at a concentration of approximately 50% w/w aqueous solution as this is approximately the eutectic point for such a composition. The freezing point of 50% w/w aqueous potassium formate or acetate is −60° C. (−76° F.). However, when the concentration of potassium formate or potassium acetate is increased to above this level, the freezing point becomes significantly higher. For example, a 65% w/w solution of potassium formate or potassium acetate has a freezing point of approximately −22° C. (−8° F.) and a 70% w/w solution has a freezing point of approximately −10° C. (14° F.). Therefore potassium acetate or potassium formate solutions of such high freezing points exhibit poor performance as de/anti-icer compositions and their practical use is limited. With such high freezing points there is a serious risk of the potassium acetate or potassium formate solution freezing in storage tanks, in the associated de-icing equipment or freezing on the ground during cold weather.
Unfortunately, limiting the de/anti-icer compositions to a maximum 50% w/w potassium formate or potassium acetate also limits the effectiveness of the compositions. Such compositions must necessarily comprise at least 50% w/w water. Because melting of ice and/or snow inherently causes further dilution, this inherent quantity of water limits the amount of further water/ice melt such compositions can accommodate before the freezing point of the resulting mixture rises above ambient temperature, that is, before the resulting mixture itself freezes. Consequently, both the “hold-over” time (i.e. the time during which the composition continues to have an anti-icing effect) of products based on 50% w/w acetate or formate compositions when applied as anti-icers, and the amount of frozen material that can be cleared per application as a de-icer, is limited. Furthermore, the presence of 50% w/w water means that the potassium acetate or potassium formate concentration gradient at the de-icer/ice boundary is relatively limited, which restricts the rate at which acetate or formate ions migrate down the concentration gradient into the crystal structure of the ice to cause it to melt.
The inventor has appreciated that potassium formate or potassium acetate ions present in standard 50% w/w aqueous de-icer compositions are also completely hydrated, negating any advantageous thermodynamic effects which could otherwise occur arising from interactions between incompletely hydrated species and water.
U.S. Pat. No. 5,064,551 describes de-icing compositions comprising potassium acetate or potassium formate, in conjunction with small amounts of phosphate and nitrite salts. The content of potassium acetate/formate can be as much as 60% w/w and compositions of 50% to 53% w/w potassium acetate/formate are stated to be preferred. Only compositions containing 50% potassium acetate are exemplified.
Embodiments of the present invention seek to provide de/anti icer compositions which can ameliorate or overcome some or all of the above problems. In particular, embodiments of the present invention seek to provide de/anti-icer compositions which can have at least one or more of the following advantages. That is, embodiments of the invention seek to provide de/anti-icer compositions which: (i) can be effective at lower temperatures, (ii) can be applied at lower application rates, (iii) can be relatively inexpensive, (iv) can have reduced environmental impact and/or (v) can be safe for applications where the compositions may come into contact with aircraft.